This application relates in general to processing metals and more particularly to an apparatus and process for removing volatile coatings from scrap metal.
Because the energy required to melt aluminum metal is considerably less than that required to extract aluminum from its ores, much of the aluminum used in manufactured goods derives from aluminum scrap--and one of the principal sources of aluminum scrap is discarded beverage cans. The typical aluminum beverage can has an organic coating, usually a lacquer, on its interior and exterior surfaces, and this coating causes a considerable amount of dross when aluminum cans are introduced into a melting furnace. In this regard, within the furnace the coating volatilizes and ignites before the can melts, and the combustion which ensues oxidizes the aluminum, thereby creating the dross which is actually an oxide of aluminum. Processors of aluminum scrap therefore usually subject aluminum beverage cans to a delacquering operation before introducing them into a melting furnace. Also, beverage cans may contain residual moisture, perhaps in the form of the beverage itself, and to ensure the safety of those operating the melting furnace, the moisture should be eliminated before the cans enter the furnace.
U.S. Pat. No. 5,059,116 and 5,186,622 disclose a system for removing volatile coatings from aluminum beverage cans while keeping dross to a minimum. Basically, the system heats the aluminum scrap in a rarefied airstream--one without enough oxygen to support combustion of the volatile coating--so that the coating volatilizies and enters the airstream. Further downstream, the volatile components in the airstream are mixed with outside combustion air and ignited to rid airstream of the volatile components and to further elevate the temperature of the airstream. The system maintains control of the critical temperature in the kiln by varying the mass flow of the air through the kiln. Not only does the temperature of the airstream remain constant where the air enters the kiln, but so does the temperature gradient of the airstream within the kiln. To control the temperature of the airstream at its entrance to the kiln, the system includes a heat exchanger which extracts heat from the airstream, thereby lowering its temperature to an acceptable magnitude at the entrance to the kiln. To be sure, the heat extracted is used to heat the combustion air that sustains the combustion of the volatiles in the airstream, but even so, the heat exchanger increases the cost of the system and increases its complexity.